The laser has been applied in surgical procedures for many years. However, in all of these applications the lack of highly precise methods or devices often resulted in damage to the encircling cells. In many applications the effect of the laser in a wide variety of wavelength regimes is due to the production of heat; however, this heat is hard to confine, so that often the zone of thermal damage is considerably larger than the actual laser spot interacting with the tissue. In other applications, tissue removal is caused by dielectric breakdown in the tissue, caused by a highly focused laser beam in a wavelength regime that is normally associated with thermal effects only. This dielectric breakdown can be carried out with high Z resolution only under stringent optical conditions that require high numerical aperture lenses with a short focal length. These conditions restrict the use of this method to regions of the body that would allow the introduction of a lens close (a few millimeters) to the surgical surface. Furthermore, this type of laser/tissue interaction also produces damaging shock waves.
In order to provide laser microsurgery that is highly precise, a laser is required that can have a variable spot size from dimensions of a few microns and which has Z-direction resolution of at least a micron. Such a laser would produce no thermal or other spatially hard to confine damage to the surrounding tissue in all three dimensions.
To meet these requirements, it would be ideal for a laser to interact with biological tissue in a photochemical rather than a thermal mode. Biological tissue is composed of molecules that are principally formed of carbon, nitrogen and oxygen. The bonds that these atoms make have energies of dissociation in a regime that corresponds to the deep ultraviolet region of the electromagnetic spectrum. There is a laser, known as argon fluoride (ArF) excimer laser, which operates at 193 nm, and which is the shortest wavelength laser that can propagate in air. It has been shown [R. Srinivasan and B. Braren, Chem. Rev. 89, 1303 (1989)] that such a laser wavelength is absorbed by biopolymeric molecules which are then raised by the radiation to a dissociative excited state. Once in such an excited state, the molecules enter a photochemical pathway in which there is direct break-up of the molecular bonds. In principle, all of the energy of the photon in the foregoing ablation process goes to break up the molecules rather than heat the material. In fact, however, excimer lasers exhibit many emissions some at longer and others at shorter wavelengths. The longer wavelengths at 248 nm and 308 nm result in ablation but with increasing thermal effects and depth of penetration. In addition these wavelengths are known to cause damage to genetic material. Available laser wavelengths shorter than 193 nm can only propagate in vacuum, but of greater importance is the fact that 193 nm is the shortest wavelength for which optical elements exist to guide and focus the beam, and such elements function in a fashion that is considerably less precise than that required for the microsurgery applications envisioned by the present invention. Specifically, lenses in this region exhibit considerable aberration, thus making their focal point too big for microsurgery. In addition to these optical limitations, there is a further problem in the use of the ArF excimer laser within biological aqueous solutions; namely, that such solutions cause a strong absorption of the 193 nm radiation. This limits the penetration of the radiation in the solution to a few microns. The instrument disclosed herein overcomes all of these problems and allows for microsurgery with unparalleled precision in the X, Y and Z direction without heat damage to the surrounding tissue.
The principal application of the ArF excimer laser in medicine is in the field of refractive surgery. In this area there are two approaches, both of which have to work within the boundaries of the limitations discussed above. The application of refractive surgery uses the lack of heating of the surrounding tissue to remove layers of the cornea with the aid of a slit that produces a line image of the laser or with the aid of a variable aperture that allows the form of the corneal refraction to be changed. Actually, this application is not microsurgery and in fact there is presently no microsurgery performed with the ArF excimer laser. Furthermore, such applications do not encounter the problem of ablating tissue in a surrounding liquid which is encountered in the majority of microsurgical operations.